It's a combination of a number of factors: planes landing with many passengers, shortage of taxis, back log due to bad weather.

If I'm landing at Logan, and we've had bad weather in this area, I'll prearrange transportation from the airport to my apartment. If it's been nice out, and I expect things will go smoothly, then I'll take the silver line into the city. I've been wrong, and have ended up in this long taxi line.